From practice as well as from a certain number of specifications, wiper systems for motor vehicles are known, in which during the operation of the pendulum-motion of the wiper arm and thus of the wiper blade carried by the wiper arm a motion of the wiper blade is overlapped essentially in its longitudinal direction. This shift of the wiper blade is achieved by changing the effective length of the wiper arm. For this purpose the wiper arm can telescopically be constructed and can comprise a piston which by means of a gear is shifted with respect to a guide housing that moves only in pendulum-motion. Such a construction is known, for instance from the DE-PS 34 05 131, the EP-OS 0 218 192 or the EP-OS 0 250 294. As to another construction, as it is shown in the DE-OS 34 05 677, more or less only the link between the wiper blade and the wiper arm can be moved with respect to the wiper arm.
As to the wiper systems described in the specifications mentioned above, the motion for changing the effective length of the wiper arm via a coupler is derived form a rotary motion of a crank which is non-rotatably seated on a stroking shaft coaxially running with respect to the wiper shaft.
From the DE-PS 34 05 131 and the EP-OS 0218 192, it is known to drive the stroking shaft continuously in the same direction of rotation during the operation of the wiper shaft. Therefore, in one half of a wiping cycle it rotates in the same direction and in the other half of a wiping cycle it rotates in the opposite direction to the wiper shaft. The consequence thereof is that the wiping areas of the wiper blade when being moved forward and backwards are different.
As to the wiper system of the DE-OS 34 05 677, the stroking shaft is driven via a stroking swivelling lever in the opposite direction each to the wiper shaft. For this purpose the stroking swivelling lever is connected to an intermediate lever via a coupler, which intermediate lever has an axis of rotation fixed to the motor vehicle. A device rod is linked both to the intermediate lever and to a wiper shaft swivelling lever fastened to the wiper shaft, which wiper shaft swivelling lever is driven by an electric motor via a crank drive. Due to the confrontation of the wiper shaft and of the stroking shaft, already small swivelling angles to the stroking crank seated on the stroking shaft lead to a big change of the angle between the stroking crank and the wiper arm and thus to a big change of length of the wiper arm.
It is known from the EP-OS O 250 294 to change the direction of rotation of the striking shaft within half a wiping cycle. Thus, the stroking shaft rotates both in the same and in the opposite direction to the wiper shaft within half a wiping cycle. The stroking shaft is driven via a stroking swivelling lever being part of a crank drive, the other crank of which is connected with the stroking swivelling lever and is formed by a pinion meshing with a stationary toothed ring. Such a construction is very expensive because of the pinion and of the toothed ring, for the pinion and the toothed ring are to mesh with each other with very little play in order to work quietly.